1. Field of the Invention
The present invention relates to an aqueous emulsion, and more particularly to an aqueous emulsion applicable to an effective component of a water- and oil-repellent agent, etc.
2. Description of Related Art
Conventional water- and oil-repellent agents including water-soluble organic solvents such as acetone, ethyl acetate, etc. have a distinguished freeze-thaw stability, but still have such problems as contamination of working circumstances due to use of organic solvents or contamination of effluent water with organic solvents, resulting in an increase m BOD and COD, etc.
"Freeze-thaw stability" is one of test items, which determines such a property of an emulsion of a water- and oil-repellent agent as return to the original state or not when the emulsion is once frozen and then returned to room temperature. Poor freeze-thaw stability indicates occurrence of such phenomena as precipitation, increased viscosity or sometimes solidification during the transport in cold districts. Once denaturing of the emulsion occurs due to such phenomena, the commercial value of the water- and oil-repellent agent will be entirely lost.
The following water- and oil-repellent agents including water-soluble organic solvents have been so far proposed.
JP-A-7-173772 discloses a water-emulsifiable, fluorine-based water- and oil-repellent agent which comprises 100 parts by weight of polyfluoroalkyl group-containing polymer and 1 to 20 parts by weight of glycol. In the conventional emulsion polymerization to form water-dispersible, water- and oil-repellent polymers, at least 50 parts by weight of a low boiling organic solvent such as acetone, ethyl acetate, etc. is used per 100 parts by weight of a monomer mixture to improve the compatibility of polyfluoroalkyl group-containing monomers with an aqueous medium or with other comonomers, where the residual organic solvent, if any, lowers the color fastness. By using a smaller amount of glycol, the disadvantage can be improved, but the glycol as an organic solvent still remains in the resulting water- and oil-repellent aqueous emulsion.
JP-A-5-263070 discloses that emulsion polymerization of fluoroalk-yl group-containing monomers is carried out in an aqueous solution containing a specific glycol ether or glycol ester. It is discloses that the proportion of these glycols to be used can be reduced to about 10 to about 30 parts by weight per 100 parts by weight of the monomer mixture in contrast to a proportion of the ordinary organic solvent of about 60 to about 100 part by weight, but the glycols as an organic solvent also still remains in the resulting water- and oil-repellent aqueous emulsion.
JP-A-6-17034 discloses a water- and oil-repellent aqueous latex where polyfluoroalkyl group-containing polymers are dispersed in an aqueous medium containing a glycol ether-typed solvent. It is disclosed that a proportion of the glycol ether-typed solvent to be used is about 50 to about 5% by weight on the basis of total weight of the emulsion including water, and thus a considerably large amount of the organic solvent is used.
JP-A-5-279541 discloses that various organic solvents such as acetone, etc. are present in the emulsion polymerization to form fluorine-containing copolymers, where it is stated that a considerable portion of the solvent can be made to remain in the resulting aqueous dispersion, but a complicated, expensive procedure for removing the solvent at 40.degree. to 90.degree. C. in vacuum is used from the viewpoints of safety and industrial sanitation.
JP-A-5-17538 discloses formation of an aqueous emulsion by once emulsifying and dispersing perfluoroalkyl acrylate, carboxyl group-containing .alpha., .beta.-ethylenic unsaturated monomer and hydroxy group-containing .alpha., .beta.-ethylenic unsaturated monomer into water to form particles having particle sizes of not more than 0.3 .mu.m, followed by polymerization. Though the disclosed procedure uses no organic solvent, not only a complicated means of conducting ultrasonic irradiation for 60 minutes while bubbling the emulsion having an average particle size of about 1.mu.m with a nitrogen gas is used to make the particle size not more than 0.3.mu.m, but also there is a possibility of precipitation when only a small amount of a cationic component is mixed therein during the water- and oil-repellent processing such as dipping, etc. because the resulting copolymers partially contain carboxyl groups.